Due to the growth and the increasing interest of mass-market users in respect of exchanges of digital data supporting audiovisual programs or files over the INTERNET network, numerous services, aimed at catering for the broadcasting of such audiovisual sources, have been proposed hitherto.
Among them may be mentioned the following main services:
WEB TV Service
Such a service makes it possible, by consulting a WEB site, to retrieve a URL address (Uniform Resource Locator address) so as to request an audiovisual content directly. This content is not broadcast but duplicated and transmitted by the source as many times as there are users connected.
The consulting of a televised program transmitted in this mode involves the user connecting up each time to the aforesaid WEB site, so as to request a content currently being broadcast.
The information on the content of the program and the description of the mode of broadcasting is implicit or provided by the WEB site on request.
Multicast Over the MBone Service
With reference to FIG. 1a, within the context of this service, the SDP data structure available for each audiovisual source is used (1) to find (2) all the information necessary for placing in receive mode by the IGMP protocol, RFC 2236: Internet Group Management Protocol, for an audiovisual source AV with the appropriate protocols and decoders.
An exemplary SDP data structure is given hereinbelow:                v=0        o=mhandley 2890844526 2892842807 IN IP4 126.16.64.4        s=SDP Seminar        i=A Seminar on the session description protocol        u=http://www.cs.ucl.ac.uk/staff/M.Handlry/sdp.03.ps        e=mjh@isi.edu(Mark Handley)        c=IN IP4 224.217.12/127        t=2873397496 2873404696        a=recvonly        m=audio 49170 RTP/AVP 0        m=video 51372 RTP/AVP 31        m=application 32416 udp wb        a=orient: portraitDVB SI Service        
Telebroadcast digital broadcasting in Europe, either by satellite or by cable or over the airwaves, is based on the specifications drawn up by the DVB forum. The DVB/SI specification defines the way of broadcasting, in a television multiplex, the information necessary for the acquisition of each of the programs broadcast in this multiplex. This mode implies that the reception terminal be capable of receiving the entire multiplex, for example 40 Mbits/s for a satellite, so as to be able to extract the program or programs chosen by the user.
IP SAT Service
In the case of this service, a means of broadcasting, the satellite, has to be used to transmit at high bit rate data destined for one or more destination users, usually on the request of users. The broadcast is then activated by the requesters.
IETF Service
The IETF has defined a data structure which makes it possible to convey information on the contents and all the information useful to the terminals to acquire them, either by downloading, or by placing oneself in receive mode for a broadcasting channel (RFC 2327: SDP: Session Description Protocol).
The aforesaid services consist, essentially, in placing in the SDP structure retrieved, either from a WEB site, or from an electronic message, all the information necessary in order to be able to place oneself in receive mode for the audiovisual source. Such a modus operandi assumes that the aforesaid information does not change during transmission.
Represented in FIG. 1b is an audiovisual source AV connected to the INTERNET network and that broadcasts in multicast a televised program on several streams.
Several techniques can be used to serve a dispersed population of terminals with modes of hookup of these terminals that are very diversified, such as for example telephone network with modems having various bit rates, ADSL, cable, satellite, ISDN, or the like.
Either the televised program is broadcast at different bit rates at 20, 40 or 60 kbits/s for example, or the signal is split into several streams, a base stream at 20 kbits/s and one or more complementary streams at 10 or 20 kbits/s that make it possible to improve the quality of the signal conveyed by the base stream for a broadcast to terminals linked to the switched telephone network. The audiovisual source AV always transmits toward the INTERNET network, whether or not there is a taker terminal for this transmission. It may be concluded that, at the start of the transmission, all the streams transmitted by the audiovisual source stop in the first appliance of the network, failing a request of a terminal for example.
When a terminal is configured to receive the televised program, it uses the information contained in the SDP data structure to obtain the address of the audiovisual source, its mode of broadcasting and of coding. It can then proceed to the choice of the digital stream or streams that it is capable of receiving and transmit a request to receive the audiovisual source via the IGMP protocol, providing the broadcasting address of the chosen channel. The operation is repeated as many times as the terminal has chosen this stream.
A path is then constructed by switching between the receiving terminal and the closest FAI router which is already receiving the digital stream supporting the requested telebroadcast program originating from the audiovisual source.
The aforesaid prior art solution has the drawback of the absence of synchronization between the moment at which the terminal retrieves the SDP data structure and the moment at which the terminal places itself in receive mode for the audiovisual source. Thus, for a regularly televised program broadcast on INTERNET, the user connects up once to the WEB site of the station broadcasting this televised program so as to retrieve the SDP structure, and, subsequently, places his terminal in receive mode for this audiovisual source, each time he so desires, without reconnecting to the aforesaid WEB site.
Any change of the conditions of reception of the terminal, such as subscription, access network or the like, renders the information contained in the SDP structure null and void.
Any change of the mode of broadcasting of the audiovisual source, such as a change of format, transmission title, indication of end of transmission, mode of scrambling or the like, renders the information available in the SDP structure originating from a WEB site null and void.